The easiest way would be to extend the related html files. Disadvantage, they will be replaced by template updates and the work has to be done again. However, due to the simplicity I will explain this way.
Impressum/Copyright
You could simply insert the following into footer.html after line 29:
<p>This Wiki is hosted and ruled by Policies of [pmueller].<br />
By using this wiki you accept these rules.
<a href="http://[your domain]/impressum.html" class="external ">-> Impressum</a>
</p>
Just to explain the the code above (you may use something else:
1. replace
[pmueller] with your company or organization name.
You may use a link to the start of your homepage (<a href="http://[your domain]" class="external ">Your company</a>).
2. create an
impressum.html file (if necessary) and store it into your domain path.
It can be the same as you may use for the while domain. You have to add the path to your domain
if the file is not stored flat in the base directory of your homepage.
Logo
The relevant part in header.html is line 23.
$logo = tpl_getMediaFile(array(':wiki:logo.png', 'images/logo.png'), false, $logoSize);
It tells the template where to look for the logo image. There are two possibilities. Either you store your logo with the name logo.png within the paths or you modify the code-line towards your logo filename.
The path are:
dokuwiki/data/media/wiki/logo.png
dokuwiki/lib/tpl/dokuwiki/images/logo.png